An optical recording medium capable of recording and/or reproducing NTSC signal data of up to four hours on one side thereof has been proposed for optical discs of the next generation. This proposed optical recording medium allows a home-use video disk recorder to record and reproduce data for up to four hours, thereby functioning as a new recording medium which can be replaced with a cassette used in a current video cassette recorder (VCR).
Since the above optical recording medium has the same shape and size as a compact disc (CD), even a user accustomed to easy handling and operability of a CD will feel similar ease in use of this medium. Moreover, if the fast access feature of this optical recording medium is utilized, an optical disc apparatus is provided which not only serves as a small-sized and easily operable :recorder but also has various functions such as video recording, playback, a trick play, edition, etc. that can be performed instantaneously.
It has been found that such an optical disk apparatus requires an optical disk medium having a storage capacity of at least 8 GB or more. However, optical recording mediums currently available do not have storage capacities of 8 GB or more.
A digital versatile disc (DVD) only has a storage capacity of 4.7 GB (ROM) when a wavelength .lambda. is 0.65 .mu.m and a numerical aperture (N.A.) is 0.6.
If a higher storage capacity is desired without any change of signal format, such as an error correction code (ECC), a modulation system or the like, establishment of the following equation (1) is required for obtaining of a storage capacity of 8 GB or more. EQU 4.7.times.(0.65/0.60.times.N.A./.lambda.).sup.2 .gtoreq.8 (1)
According to the above equation (1), N.A./.lambda..gtoreq.1.20 must be established. As a result, it is therefore necessary to set the wavelength .lambda. of the laser light utilized by the optical disk apparatus shorter or to set the N.A. higher wherein N.A. represents the numerical aperture of an optical head device of the optical disk apparatus.
In order to satisfy the above condition, if the value of N.A. is set higher, it is necessary to decrease the thickness of the light transmissive layer of the optical recording medium through which irradiated reproduction light is transmitted. This reduced thickness is necessary to compensate for a consequent reduction in the allowance for a tilt angle wherein the disk surface is displaced from a surface perpendicular to the optical axis of the optical pickup. Specifically, the tilt angle is easily affected by optical aberrations resulting from the thickness of the base of the optical recording medium.
For a similar reason, unevenness of the thickness of the light transmissive layer must be reduced to no more than a predetermined value.